1. Field of the Invention
The present invention relates to a circuit for switching a microwave signal to a first output or a second output. This signal is preferably a power signal. It also relates to a switching device including several switching circuits.
This switching circuit and switching device are designed notably for the switching over of the supply to antennas. It enables a signal delivered by the final power stage of a transmitter to be sent towards an antenna chosen from among several antennas.
2. Description of the Prior Art
In certain applications, a power signal coming from a final stage of a transmitter has to be directed towards a particular use, for example an antenna illuminating a particular illumination zone. Telecommunications satellites may include antennas successively illuminating distinct zones. A signal re-amplified in a transponder has to be oriented towards the antenna corresponding to the zone to be illuminated, as a function of the traffic to be conveyed.
In certain radar systems, a signal coming out of a transmitter has to be sent, sometimes to a so-called main antenna and sometimes to a so-called control antenna.
In existing switching circuits, the switching is done at output of the final power stage of the transmitter by means of a commutator or changer switch.
This changer switch may be mechanical or electromechanical, or it may be a ferrite switch. It may be controlled by hand or it may be remote-controlled. In any case, it has to switch over signals having high peak power values of up to several kilowatts and mean power values ranging from several tens to several hundreds of watts.
These standard switching circuits have several drawbacks.
At high power, the transmission losses in the changer switch may attain levels of the order of one decibel. For an antenna to radiate a given power value, the final stage of the transmitter should give about 25% of power in addition to the power radiated by the antenna. The transmission losses in the changer switch call for the use of a special cooling device.
The reliability of the power changer switches decreases, firstly, with the level of power brought into play and, secondly, with the number of switch-over operations per unit of time.
Should there be a failure in the power stage of the transmitter, there is a standard method used to avoid any interruption in the transmission: this method uses two identical amplification chains. Only one chain is in operation while the other chain is passive, in a state of redundancy. An additional changer switch has to be used to make the passive amplification chain active if need be. The additional changer switch doubles the transmission losses. Each of the amplification chains must give about 60% of additional power with respect to the power radiated by the antenna, if it is considered that each changer switch has losses of one decibel.
In satellite telecommunications systems, the levels of power transmitted by the onboard transponder do not presently go beyond 300 watts CW. However, the losses of the changer switches and their limited reliability greatly limit the performance characteristics of such systems.
On the ground, the transmitted power values are highly variable and may range from some watts to some megawatts.